The case pursued was based on those arguments. I'm not surprised that this court chose to rule it narrowly, given the ideology of the justices. Still, it's important that this decision dramatically limits the amount of enforcement states can do in this area. The reason for this is because the goal is to make certain states openly hostile areas to immigrants, or really to anyone perceived as an immigrant. On a national scale you have far less chance of slipping that through. Though I'll grant that it would go through should Romney win and the Republicans take the Senate, I'd much rather see this fought out on a national level than in each state legislature.

EDIT: Forgot to mention, it's worth noting that civil rights challenges are still being pursued against this law and this ruling didn't shut the doors on them.